This research demonstration project is a multisite study focusing on the long-term prevention of serious and chronic conduct problems in adolescence and early adulthood. The preventive intervention targets third-grade boys and girls who are at significant risk for delinquent and other serious antisocial and mental health problems in adolescence and adulthood as evident in high rates of cross-situational aggressive-oppositional behavior problems. The overall plan is to: (1) identify cross-situationally aggressive children at risk, (2) implement a theory-driven, intensive, multi-component, three-year-long preventive intervention, and (3) through childhood, adolescence, and early adulthood, compare the preventive intervention with a control intervention. During Phase I, the first half of the project sample is randomly assigned to the experimental or control intervention, each lasting three years. During Phase II, the interventions are conducted with additional children to complete the overall sample and all children are followed into early adolescence. Phase III involves longitudinal follow-up of the entire sample through adolescence and early adulthood. Driving the experimental intervention Comprehensive Preventive Intervention) is a theoretical model which postulates that effective nonverbal and verbal communication at home, at school, and with peers promotes children's social, achievement and affective competence, provides gratification, and prevents behavior problems. The core component of the Comprehensive Preventive Intervention is Family Skills Training, which is designed to change the interaction and communication patterns of all family members while altering the target child's aggressive-oppositional behavior, impulse control, and affect regulation. Family Skills Training is further strengthened by the Classroom Achievement System to improve classroom communication and achievement and to reduce child behavior problems at school, and by Peer Communication Groups to improve child communication with peers and reduce behavior problems with peers. The Control Intervention consists of the Classroom Achievement System by itself. The research demonstration project is characterized by several critical features including (1) matched random assignment to intervention, (2) inclusion of girls as well as boys in the at-risk sample, (3) a sample drawn from diverse populations located in a medium-sized Southern city, a large Western city, and a semi-rural Midwestern community, (4) stringent criteria for at-risk status, (5) careful standardization of intervention and evaluation procedures coupled with attention to treatment fidelity, and (6) deployment of multisource, multimethod, and multisetting measures at repeated intervals. Measures will evaluate impact of (a) during intervention on increases in skills and decreases in problems, (b) immediately after intervention on competence, presumed to spontaneously improve as skills accelerate and problems decelerate, (c) immediately after intervention on prevention of conduct problems, and (d) long after intervention on conduct problems and molar life outcomes in adolescence and early adulthood. The project is expected to provide a rigorous demonstration of the impact of an intensive and multifaceted early preventive intervention on the subsequent development of serious and chronic conduct problems.